This invention concerns digital radio communication systems. It concerns, in particular, the methods of modulation implemented in such systems.
Digital modulation is generally designed to combine the following three requirements: a high transmission rate, minimum spectrum occupancy and a low transmission error rate under various operating conditions.
Various methods were proposed in the past with a view to achieving a high transmission rate on a channel with reduced spectral bandwidth (transmission rate exceeding 1 bits/Hz).
The first group of methods uses multi-level frequency modulation as a basis, together with adequate filtering of the modulating signal (e.g. Gaussian filtering used with GMSK modulation) in order to reduce adjacent channel interference. These methods have the advantage that they are easily applied, and result in modulated signals of constant envelope. They consequently permit transmitters to be fitted with power amplifiers which operate in the saturated state. These amplifiers are readily available, cheap and very efficient. However, in order to comply with constraints relating to adjacent channel interference, the modulation index must be limited considerably, and the modulating signal thoroughly filtered. This causes the symbol spacing to be reduced, and this adversely affects the noise immunity of the modulation. In other words, the sensitivity of the radio receivers is limited.
Another group of methods uses phase-shift keying (PSK) and, if necessary, differential phase shift keying (DPSK) as a basis, and the resulting signal is filtered to ensure that standards relating to adjacent channel interference are complied with. In general, a filter satisfying the Nyquist criterion is used in order to limit inter-symbol interference. These methods generally provide satisfactory sensitivity at the expense of a large variation in the amplitude of the radio signal. Very linear amplifiers are therefore necessary, and they are difficult to design and set up. In addition, they are generally inefficient, and this seriously affects the autonomy of mobile stations. A non-linear amplifier can be used in conjunction with a linearizing method, but such method complicates a transmitter very considerably if there are large envelope variations.
Other solutions have also been proposed, e.g. in U.S. Pat. Nos. 5,642,384 and 5,311,552, where an appropriate choice of a constellation and of a coded modulation process prevents transitions in the constellation for which the phase change is relatively large. This permits the variation in amplitude of a radio signal to be reduced to values compatible with the characteristics of amplifiers which are easier to design. However, the reduction in amplitude is achieved at the expense of a considerable reduction in the symbol spacing, which is very difficult to compensate by coding gains, in particular in the error rate range of the greatest importance to speech communications, i.e. for bit error rates (BER) of the order of 10xe2x88x922, especially when the channel is affected by fading (Rayleigh fading).
An object of the present invention is to propose a digital modulation group permitting joint optimisation of noise immunity, even in a channel affected by fading, adjacent channel interference, and variation in amplitude of the radio signal.
The invention thus proposes a digital modulation process wherein the successive symbols of a digital stream are converted into phase increments, an accumulated phase is obtained by adding the successive phase increments, a modulating phase is obtained by filtering the accumulated phase, a complex signal is produced whose argument represents the modulating phase, two phase quadrature radio waveforms are respectively modulated on the basis of said complex signal, and a radio signal resulting from a combination of the two modulated waveforms is transmitted. According to the invention, said complex signal is digitally filtered, and digital signals obtained from the real and imaginary components of the digitally filtered complex signal are converted into analog form before being respectively subjected to anti-aliasing analog filtering and then mixed with the two radio waveforms.
Said digital signals obtained from the real and imaginary components of the digitally filtered complex signal typically consist of the real and imaginary components themselves. However, if an amplifier linearizing process is used, by pre-distortion for example (see European patent application No. 0 797 293), the real and imaginary components may be subject to correction before being converted into analog form. The use of a linearizing process is not included directly in this invention. In many cases, the invention will permit such a process to be dispensed with. In other cases, it will permit the use of such processes to be simplified considerably (for example, by not taking account of phase changes), in view of the small variations in the signal envelope permitted by an appropriate choice of parameters for filtering the accumulated phase and said real and imaginary components. The criteria for this selection will be specified further on.
The invention permits digital radio communication systems, in particular professional radio communication systems, to be implemented in accordance with applicable standards relating to adjacent channel interference, and provides unequalled sensitivity and thus radio range, using power amplifier components which are readily available on the market and have a high power efficiency.
Another aspect of the invention relates to a digital modulator, including means for converting successive symbols of a digital stream into phase increments, a summator which accumulates the successive phase increments to produce an accumulated phase, a phase filter receiving the accumulated phase and producing a modulating phase, means for producing a complex signal whose argument represents the modulating phase, and a modulator for respectively modulating two phase quadrature radio waveforms on the basis of said complex signal, and for transmitting a radio signal resulting from a combination of the two modulated waveforms, the modulator comprising a digital filter to which the complex signal is applied, digital-to-analog converters respectively processing the digital signals obtained from the real and imaginary components of the digitally filtered complex signal, anti-aliasing analog filters receiving the output signals from the digital-to-analog converters, and two mixers each receiving one of the two radio waveforms and the output signal from one of the two anti-aliasing filters.
According to another aspect of the invention, there is provided a digital modulation process, comprising the steps of:
converting successive symbols of a digital stream into phase increments;
obtaining an accumulated phase by adding the successive phase increments;
obtaining a modulating phase by filtering the accumulated phase;
producing a complex signal having an argument representing the modulating phase;
modulating two quadrature radio waveforms, respectively, on the basis of said complex signal; and
transmitting a radio signal resulting from a combination of the two modulated waveforms,
wherein the step of obtaining the modulating phase includes the step of filtering the accumulated phase in a phase filter with a finite impulse response having both positive and negative terms,
and wherein the phase filter has a frequency response providing an attenuation substantially higher than 3 dB for a frequency excursion of 1/(2Ts), where Ts is the symbol period in the digital stream.
The response of such phase filter advantageously corresponds to a time characteristic having the form:
g(t)=Sinc(xcex1xe2x80x2t/Ts).Sinc(xcex2xe2x80x2t/Ts).exe2x88x92(xcfx80xcex3xe2x80x2t/Ts)2,
where xcex1xe2x80x2, xcex2xe2x80x2 and xcex3xe2x80x2 are real coefficients, and Sinc( ) is the cardinal sine function. When Ts=125 xcexcs and each symbol of the digital stream consists of two bits, with phase increments of xe2x88x92xcfx80, xe2x88x92xcfx80/3, xcfx80/3 or xcfx80, preferred values of the response coefficients are xcex1xe2x80x2≈0.77, xcex2xe2x80x2≈0.5 and xcex3xe2x80x2≈0.